A key component of many physiological and pathophysiological processes is the adhesion of leukocytes to the endothelium in the fluid dynamic environment of the circulation. This adhesion involves multiple receptor-ligand pairs which may interact simultaneously and in a functionally overlapping fashion. One approach to elucidating the biophysical and molecular mechanisms which govern this complex cascade is to study the interaction of one particular type of leukocyte adhesion molecule with the endothelium in the absence of other adhesive events. We will use such an approach to investigate hypotheses concerning leukocyte adhesion to the endothelium. Specifically, we will coat spherical 10 mum diameter polystyrene microspheres with the leukocyte ligand P-selectin glycoprotein ligand-1 (PSGL-1), the beta2 integrin Mac-1 (CD11b/CD18) or the beta2 integrin LFA-1 (CD11a/CD18). Previous studies have demonstrated that microspheres coated with PSGL-1 qualitatively recreate cellular attachment and rolling on P-selectin. An unresolved issue concerning the PSGL-1 microsphere system (and this approach in general) is the extent to which ligand coated microspheres quantitatively recreate cellular adhesive events. The first aim will focus on this issue. We will make a detailed quantitative comparison between PSGL-1 mediated microsphere adhesion to P-selectin to that of PSGL-1 mediated neutrophil adhesion to P-selectin. This study will give insight into the cellular and molecular mechanisms which govern attachment and rolling. The second aim focuses on adhesion mediated by the neutrophil beta2 integrins, Mac-1 and LFA-1. Based on several observations, it is postulated that beta2 integrin-E-selectin but not beta2 integrin - P-selectin molecular interactions can mediate adhesive processes which may be involved in neutrophil adhesion to the endothelium. To explore this hypothesis, the adhesion of Mac-1 and LFA-1 coated microspheres to E- and P-selectin under in vitro flow attachment and detachment conditions will be studied to determine if beta2 integrin- E-selectin and/or beta2 integrin - P-selectin molecular interactions can mediate adhesive processes.